In CFB boilers, generation of hot steam from feed water takes place in several stages, for example, by means of heat exchangers arranged in the backpass of the boiler, by means of water tube panels of the furnace and backpass walls, and in heat exchange chambers arranged in the external hot circulation. As larger and more efficient CFB boilers are developed, heat exchange chambers in the external hot circulation become increasingly important. Therefore, ways must be found to advantageously provide the boilers with heat exchange chambers that are capable of producing a sufficiently high heat transfer power, while still operating flexibly in various operating conditions.
OTU boilers have the advantage of not needing a density different between water and steam, to provide the driving force for water circulation which cools the evaporator tubes of the furnace walls of the boiler. Instead of the density difference, the feed water pump of the boiler acts as the driving force for the water circulation. Therefore, in OTU boilers, it is possible to heat the steam to high temperatures at pressures above the critical point of water (220 bar), which improves the efficiency of the water vapor generation process of the boiler. In suspension-fired boilers, in operation and having capacities of about 1000 MWe, in which the temperature of the flue gas exiting the furnace may be about 1300° C., the achieved end temperature of steam at about 300 bar pressure has been 610° C. In CFB boilers, in which the furnace temperature is typically 850° to 900° C., achieving corresponding steam values and, especially, a high reheat temperature, e.g., 620° C., calls for new solutions in the designing of boiler heat exchangers.
A heat exchanger has a high efficiency when a large amount of solids, having a high inlet temperature and low outlet temperature, flows through it. In general, it is possible to raise the efficiency of the heat exchanger by increasing its heat exchange surface, which requires that the volume of the fluidized bed in the heat exchange chamber is large enough. Increasing the height of the fluidized bed increases the pressure loss of the fluidizing gas, and increasing its width and depth may lead to disadvantageous solutions in view of the structure or space consumption. To avoid these problems, it is advantageous to use at least two separate heat exchange chambers, instead of one large heat exchange chamber.
U.S. Pat. No. 5,275,788 discloses a heat exchanger of a CFB boiler comprising two heat exchange chambers arranged in association with the furnace wall, one on top of the other, but in parallel, in view of the particles flow. Desirable portions of the solids separated from the boiler exhaust gas by means of a particle separator may be introduced into these heat exchange chambers. With this kind of a heat exchanger, the solids to be introduced into both heat exchange chambers have the same temperature, and the end temperature of the solids remains very high. Thus, the heat exchange efficiency of the heat exchanger, and the adjustability of the heat exchange efficiency, may be inadequate, especially at low loads.
U.S. Pat. No. 5,537,941 discloses a heat exchanger with two stacked sections, an upper and a lower section, connected with each other in series, both sections having two heat exchange chambers connected in parallel. Both the upper section and the lower section also comprise a bypass channel through which a portion of the solids entering each section may be passed in a non-cooled condition, past the heat exchange chambers into the solids exiting the section. The adjustability of this kind of a heat exchanger is quite good, but even here, the efficiency and flexibility of the heat exchanger are not necessarily sufficient in all operational conditions of the boiler.